1. Field of the Invention
The present invention relates to a polyvinyl alcohol film. More precisely, the invention relates to a polyvinyl alcohol film that is useful for a raw material for polarizing films of good polarization performance, durability and hue.
2. Description of the Related Art
A polarizer having the function of transmitting and blocking out light is a basic constitutive element in liquid crystal displays (LCD), along with the liquid crystal that functions as a switch for light therein. Regarding their application field, LCDs were almost for small-sized instruments such as electronic calculators and wristwatches in the early days at the beginning of their development, but in these days, their applications are much expanding for other various instruments such as laptop personal computers, word processors, liquid-crystal projectors, navigation systems for automobiles, liquid-crystal televisions, personal phones, as well as measuring instruments for indoor and outdoor use, etc. With the expansion of the field of such LCD applications, neutral gray polarizers of better polarization performance and durability and also better hue for better color display quality are desired than those of conventional ones.
In general, a polarizer is constructed by laminating a polarizing film with a protective film such as a triacetyl cellulose (TAC) film or a cellulose acetate butyrate (CAB) film, for which the polarizing film is prepared by monoaxially stretching and coloring a polyvinyl alcohol film (polyvinyl alcohol will be hereinafter abbreviated as “PVA”; and the polyvinyl alcohol film will be as “PVA film”), or coloring and then monoaxially stretching it, followed by treating it with a boron compound for fixation (as the case may be, any two treatments of coloration, stretching and fixation may be effected at the same time).
Applications of liquid-crystal displays are expanding these days to high-definition personal computer monitors and televisions for domestic use, and they are much desired to have higher contrast, longer life and better color reproducibility than conventional ones. To that effect, liquid-crystal displays are required to have increased high-level performance, and the polarizing films for them are required to have improved polarization performance for higher contrast and increased durability for longer life, and, in addition, they are further required to have good hue for improved color reproducibility.
For improving the optical performance and the durability of polarizing films, various investigations have heretofore been made principally for the matters as to how the structure of PVA and that of PVA films for polarizing films are designed and how the condition in fabricating polarizing films is controlled. However, with the increasing the performance of liquid-crystal displays, the necessity of improving the performance of polarizing films for them is much increasing, and the methods that have heretofore been investigated in the art could not produce polarizers that satisfy all the necessary requirement.
For example, regarding the study of PVA structures for polarizing films, a proposal has been made for increasing the degree of polymerization of PVA to thereby improve the polarization performance and the durability of polarizing films of such high-polymerization PVA (JP-A 1-84203). However, this is problematic in that, if the degree of polymerization of PVA is too high, the viscosity of the polymer liquid to be formed into films is too high and the polymer liquid is therefore difficult to filter, and, if so, the films formed may be often contaminated with impurities, and liquid-crystal displays that comprise such a contaminated film may have a drawback of brightness failure. In high-definition liquid-crystal displays that are further improved these days, the presence of impurities of even around 10 μm in size is problematic since the minimum size of the display dots is a few tens microns. In that situation, however, if the degree of polymerization of PVA for polarizing films for such displays is too high, it is extremely difficult to remove such small-size impurities from the polymer liquid for the films.
Other proposals have been made for modifying PVA structures for better polarizing films. For example, one comprises introducing ethylene into PVA (JP-A 5-100115, 8-188624), and another comprises reducing the 1,2-glycol bond content of PVA (JP-A 3-175404). Even in these methods, however, it is still difficult to produce polarizing films that satisfy the recent high-level requirements of good polarization performance and durability.
On the other hand, for improving the display quality in liquid-crystal displays, it is desired these days to protect polarizing films from discoloration. For this, for example, known are a method of adding a specific colorant to an adhesive for bonding a polarizing film and a protective film to thereby control the color of the resulting polarizer (JP-A 2001-311827); and a method of stretching a PVA film with controlling the phase difference (JP-A 2001-91736). Heretofore, however, no one knows any method that is effective for preventing the discoloration of polarizing films from the viewpoint of the structure of PVA for the films.
An object of the present invention is to provide a polyvinyl alcohol film useful as a raw material for polarizing films, of which the advantages are that their polarization performance and durability are good, they have little change in hue and they do not contain impurities of a size that may worsen the image quality in liquid-crystal displays.